This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-166878, filed Jun. 14, 1999; and No. 2000-123969, filed Apr. 25, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an image processor and a color image processor for dithering multi-tone-level input image data to convert the data into data of fewer tone levels, used in a printer, a copier, a facsimile machine, an MFP (Multi-Function Peripheral) and the like.
Conventionally, a binary image output printer employing a line head, such as a line LED (light emission diode) head, a line thermal head and a line ink jet head, forms a binary image by printing dots equal to the resolution of the head. Namely, if a line LED head is employed, dots having a size coincident with the distance between a plurality of recording elements (LED) linearly arranged in main scan direction are printed on a recording paper sheet to thereby print a binary image. If a thermal head is employed, dots having a size coincident with the distance between a plurality of recording elements (heating resistor) linearly arranged in main scan direction are printed on a recording paper sheet to thereby print a binary image. If an ink jet head is employed, dots having a size equal to the distance between recording elements (ink jet nozzles) linearly arranged in the main scanning direction are printed on a recording paper sheet to thereby print a binary image. It is also well known to shift the head slightly in the direction of the main scanning to form an image on the same paper sheet repeatedly and to thereby realize a higher resolution than that corresponding to the distance between the recording elements.
In the image forming apparatus provided with the recording head of this type, a character/line image is reproduced as a binary image simply corresponding to the resolution of the head. A graphic/photograph image is reproduced as a binary image by a halftone processing such as an ordered dither method or an error diffusion method. In the halftone processing, it is difficult to both maintain a high resolution and reproduce a high tone level. In case of the ordered dither processing using the same threshold matrix repeatedly, in particular, resolution and tone property are contradicting property. The halftone processing is also used for color characters, shading colors and the like.
Further, as the image forming apparatuses provided with a recording head as stated above, there is proposed one for modulating the printing area of one pixel (or adjusting dot size) based on multi-level image data converted by the multi-level dither processing, thereby allowing expressing one pixel with several tone levels. An example of a recording head constituted by a plurality of recording elements used for such an apparatus as well as the state of dots is shown in FIG. 33. In FIG. 33, reference symbol 1 denotes a recording head, 2 denotes an ink discharge port and 3 denotes an output dot (printed dot).
For brevity, FIG. 33 illustrates an example of the output of dots of the image forming apparatus capable for expressing one pixel with three levels including white (output 0). In addition, by arranging four or three lines of these recording elements in parallel, it is possible to record a color image of C (cyan), M (magenta), Y (yellow) and K (black) or a CMY-color image.
The image forming apparatus capable of printing such multi-level image data conducts various image processing including a color conversion processing, a UCR (under-color removal) processing and a gamma correction processing, to input RGB image data. Thereafter, the apparatus conducts a multi-level halftone processing such as a multi-level dither processing employing screen angles for the respective colors or a multi-level error diffusion processing so as to reproduce the number of tones intrinsic to a printer engine, to thereby obtain multi-level image data. The apparatus then outputs pixels having tone properties so as to enhance image reproducibility.
Generally, the ordered dither processing is relatively simple, has a high degree of freedom for configuration and has high processing speed, and the cost of the apparatus can be held down. However, it is said that the error diffusion processing is superior in image quality to the ordered dither processing. The ordered dither processing truncates quantization error in a comparison processing between input tone levels and thresholds, whereas the error diffusion processing diffuses quantization errors to peripheral pixels. Thus, they greatly differ in algorithm. As a result of the difference, compared with the ordered dither processing, the error diffusion processing can advantageously provide an output pattern having high frequency characteristics least conspicuous in light of human visibility, has a high edge holding effect and excellent image quality.
On the other hand, in a case of the halftone processing of a multi-level image output printer, it is known that the ordered dither processing and the error diffusion processing do not differ in image quality compared with the output of a binary image. This is because the truncated quantization error becomes far smaller than that of binary image data as the number of levels of the multi-level dither processing increases. In case of a high resolution printer, in particular, if the number of tones which one pixel can express is higher, the difference in image quality between the ordered dither processing and the error diffusion processing becomes less.
In addition, a method, such as a dither processing method employing fixed mask dither improved from stochastic dither or cluster dither, of realizing output characteristics comparable to that of the error diffusion processing at the same high speed as that of the ordered dither processing, is recently developed.
An ordinary binary output dither processing obtains binary output pixels by comparing input pixels with dither matrix thresholds at corresponding positions while basically, only taking into consideration a threshold array in a dither matrix on one plane. This state is shown in FIG. 34. FIG. 34 is a typical view showing a binary dither processing employing a well-known 4xc3x974 Bayer dither matrix. To simplify description, input pixels of 4-bit tone level are compared with corresponding thresholds in a dither matrix. If the input tone level is equal to or higher than the corresponding threshold in the dither matrix, 1 (black) is output and if it is lower than the corresponding threshold, 0 (white) is output, thus obtaining a binary output image in combination of 1 and 0.
As shown in FIG. 34, the dither matrix has a configuration in which a unit dither threshold matrix of, for example, 4xc3x974 (to be simply referred to as xe2x80x9cunit matrixxe2x80x9d hereinafter) is repeatedly used regularly and performs the above-described processing to all input pixels. Further, a normal output apparatus, such as a printer, often outputs a pixel similar to a circle rather than a square pixel due to the process limitations of the apparatus. The output state in this case is shown in FIG. 35. When all pixels are printed, the shapes of the printed pixels are designed to be ones completely covering ideal square pixels, i.e., circles with a diameter equal to or larger than {square root over (2)} times as large as a resolution pitch like dot xe2x80x9c1xe2x80x9d shown in FIG. 35.
On the other hand, in the multi-level dither processing, it is necessary to consider not only a plane threshold array in the above-stated basic dither matrix but also depth (pixel level) direction. For example, in case of conducting a multi-level, e.g., N-level dither processing, (Nxe2x88x921) threshold planes are required. Dither thresholds on each of the threshold planes are compared with input tone levels, to thereby obtain an N-level output image. The state of this multi-level dither processing is shown in FIG. 36 and the state of output dots is shown in FIG. 37. FIGS. 36 and 37 show multi-level outputs including 0 (white).
Normally, in the dither processing, a high-quality image can be obtained if there is some sort of correlation among thresholds on a threshold plane and that among threshold planes. Accordingly, thresholds in (Nxe2x88x921) dither matrixes are often calculated automatically based on a reference threshold array indicating such correlation.
In-the multi-level dither processing taking account of this correlation among planes, there are roughly two sequences of threshold arrays extending over the respective planes as shown in FIGS. 38A and 38B. To simplify description, FIGS. 38A and 38B show a multi-level dither processing for converting input 8-bit image data into an image of four levels per pixel (2 bits) using a 2xc3x972 reference threshold array. FIG. 38C shows the reference threshold array. This reference threshold array indicates the order of the magnitudes of thresholds arranged on a threshold plane.
The sequence shown in FIG. 38A is to determine thresholds sequentially from the first plane. For example, all thresholds on the first plane are determined and then those on the second plane are determined. In conducting a dither processing using such threshold planes, if tone level xe2x80x9c100xe2x80x9d is input, for example, a pixel at a position corresponding to xe2x80x9c1xe2x80x9d of FIG. 38C is judged to have a tone level 2, a pixel at a position corresponding to xe2x80x9c2xe2x80x9d is judged to have a tone level 1, a pixel at a position corresponding to xe2x80x9c3xe2x80x9d is judged to have a tone level 1 and a pixel at a position corresponding to xe2x80x9c4xe2x80x9d is judged to have a tone level 1.
This sequence is used in a printer such as an ink jet printer, which is basically less influenced by the appearance state, i.e., presence/absence of neighboring pixels or tone levels thereof and which can stably form an image out of independent pixels. The resolution of an image output using this sequence is very high and almost comparable to that of a printer engine. Thus, this sequence is ideal for reproducing an image by means of area modulation. However, if an input image has a uniform tone level, pixels of the same or similar size are easily filled to form an output image. Due to this, the image is susceptible to the print position accuracy or the printing accuracy, such as dot size accuracy, of the apparatus.
In the sequence shown in FIG. 38B, thresholds at corresponding positions on planes are sequentially determined from the first to the third planes. For example, thresholds at a position corresponding to xe2x80x9c1xe2x80x9d shown in FIG. 38C, i.e., xe2x80x9c20xe2x80x9d on the first plane, xe2x80x9c39xe2x80x9d on the second plane and xe2x80x9c59xe2x80x9d on the third plane are determined, and then thresholds on the first to third planes at positions corresponding to xe2x80x9c2xe2x80x9d, shown in FIG. 38C are determined. In conducting a dither processing using these threshold planes, if tone level xe2x80x9c100xe2x80x9d is input, for example, a pixel at a position corresponding to xe2x80x9c1xe2x80x9d of FIG. 38C is judged to have tone level 3, a pixel at a position corresponding to xe2x80x9c2xe2x80x9d is judged to have a tone level 2, a pixel at a position corresponding to xe2x80x9c3xe2x80x9d is judged to have a tone level 0 and a pixel at a position corresponding to xe2x80x9c4xe2x80x9d is judged to have a tone level 0.
This sequence is used in a printer, such as a laser printer and a thermal printer, which tends to be influenced by the appearance state of neighboring pixels and for which it is difficult and unstable to form an image out of independent pixels. With this sequence, a printed image has low resolution and low dot concentration. If a dither threshold array is formed as a dot concentrate type array, i.e., formed such that a plurality of dots are printed in block, an image called a halftone-dot image is formed. This halftone-dot image represents an image having points orderly arranged like a mesh while the block of dots constitute one point. Since the printer of this type is low in resolution, minor print position error in units of pixels is inconspicuous.
In either of the above two examples, all the thresholds arranged on the respective planes are automatically calculated when the reference threshold array and the threshold sequence among planes in depth direction are defined.
Furthermore, as for the formation of a color image, the tone reproduction characteristic comparable to the quality of a photograph including a highlight is becomes increasingly important in recent color printers. The reproduction of tones capable of further enhancing graininess is one of the most important technical challenges among others. Graininess indicates the inconspicuousness of dots or rough feeling in a printed image. A good graininess image indicates an image which tones change uniformly or smoothly and a bad graininess image indicates an image having conspicuous dots or roughness.
As a technique for satisfying the graininess, there is proposed a method of enhancing the graininess of a highlight using thin ink colors, e.g., light cyan and light magenta beside standard four ink colors of C (cyan), M (magenta), Y (yellow) and K (black). With this method, however, the number of recording heads and driving mechanisms increase proportionately to the number of added ink colors. If a recording head has the same number of nozzles as that of pixels on a line per color, this disadvantageously leads to cost hike.
Moreover, color printing is faced by a problem of the unevenness of colors due to slight difference in the overlapping manner of the respective colors of C, M, Y and K. As for the four color printing of C, M, Y and K, various multi-level dither methods including a halftone dot dither method employing screen angles, dispersion dither methods represented by a Bayer dither method, a cluster dither method having intermediate characteristics between that of the dispersion dither method and the Bayer dither method and the like, have been developed.
Color image printing has further a problem of unevenness of colors caused by the subtle difference in the overlapping manner of the dots of the respective colors of C, M, Y and K. For example, if the halftone dot method employing screen angles is applied to dithering, colors interfere with one another to thereby cause moire such as roseate moire. If a dispersion dither matrix such as a conventional Bayer matrix is employed, conspicuous texture appears at a specific tone part due to the low degree of freedom for the arrangement of dots. As can be seen, many problems still remain unsolved before obtaining optimum output characteristics over the entire colors or tones.
These problems with dither processing occur to both a binary output printer and a multi-level output printer employing a dither matrix. While the problems are particularly serious in the dither processing of the threshold sequence shown in FIG. 38B, they are not completely solved in the dither processing of the threshold sequence shown in FIG. 38A, either.
Additionally, while this is common to all these ordered dither processing including a cluster dither processing, periodicity tends to be easily seen over the entire tone ranges of input image data. The periodicity is particularly conspicuous in a printer with relatively low resolution.
Recently, a processing method of realizing output characteristics comparable to that of the error diffusion processing at as high speed as that of the ordered dither processing by employing fixed mask dither improved from the stochastic dither or cluster dither, is being developed. One preferred example of this method is described in Robert Unichney, xe2x80x9cThe Void-and-Cluster Method for Dither Array Generationxe2x80x9d, SPIE/ISandT Symposium on Electronic Imaging Science and Technology, San Jose, Calif. February 1993. This processing method, however, assumes only theoretical output characteristics in an ideal system and it considers the dot overlapping model of a binary printer, i.e., the manner in which neighboring dots overlap with one another at best. According to this processing method, therefore, only the improvement of output characteristics can be expected.
Even an output apparatus capable of reproducing multiple levels does not always obtain optimum output results for all tone levels of an input image. If limited to a specific tone, a conventional, well-known dither processing can obtain more visually satisfactory results than a processing which employs any other improved stochastic dither pattern.
It is an object of the present invention to improve the graininess and tone reproduction characteristics of an image outputted by an image processor and a color image processor.
The thresholds of a dither threshold plane are determined in view of the intrinsic output characteristics and the like of a binary output apparatus and a multi-level output apparatus.
According to one aspect of the invention, there is provided an image processor for converting input image data having the first number of tones into image data having the second number of tones lower than the first number by a halftone processing and for outputting an image corresponding to the image data, the image processor comprising: halftone processing means for carrying out the halftone processing using a dither threshold plane, thresholds in a predetermined threshold range corresponding to a tone range of the input image data arranged on the dither threshold plane; and image output means having intrinsic, basic tone characteristics, for outputting an image corresponding to the halftone-processed image data provided from the halftone processing means. The dither threshold plane consists of a plurality of same unit threshold matrixes, each of the unit threshold matrix consists of a plurality of sub-matrixes, an array of relatively low thresholds in the predetermined threshold range is equal among the plurality of sub-matrixes, and relatively medium thresholds and high thresholds in the predetermined range are arranged aperiodically to extend over series of sub-matrixes.
As a result, in the low tone area of an output image, low tone level (small diameter) dots are orderly outputted in a period of sub-matrix size. The graininess of the low tone parts is, therefore, improved.
According to other aspect of the invention, there is provided an image processor for converting input image data having the first number of tones into image data having the second number of tones by a halftone processing and for outputting an image corresponding to the image data, the image processor comprising: halftone processing means for carrying out the halftone processing using a plurality of dither threshold planes, thresholds in a predetermined threshold range corresponding to a tone range of the input image data are arranged on the plurality of dither threshold planes, each of the plurality of dither threshold planes including a plurality of same unit threshold matrixes; and image output means having intrinsic, basic tone characteristics and outputting an image corresponding to the halftone-processed image data provided from the halftone processing means. Each of the unit threshold matrixes consists of a plurality of sub-matrixes on not less than one dither threshold plane, an array of relatively low thresholds in the predetermined threshold range is equal among the plurality of sub-matrixes, and relatively medium to high thresholds in the predetermined threshold range are arranged aperiodically to extend over series of sub-matrixes. That is, the series of low thresholds are arranged to extend over some threshold planes according to the basic tone characteristics and arranged in a periodic and dispersed manner on the respective planes.
If the low tone region is printed by a printer having inferior tone characteristics in the low tone area, such a region is printed with dots of, for example, all tone level 1. However, according to the present invention, such a region is expressed with a concentration of dots of, for example, tone level 2. As a result, tone reproduction characteristics is improved. In that case, if the diameter of dots of the tone level 2 is sufficiently small, graininess is improved, as well.
Further, according to the invention, there is provided an image processor for converting color input image data having the first number of tones into color image data having the second number of tones by a halftone processing and for outputting a color image corresponding to the image data, the image processor comprising: halftone processing means for carrying out the halftone processing using a plurality of dither threshold planes for each color, each of the plurality of dither threshold planes including a plurality of same unit threshold matrixes; and image output means having intrinsic, basic tone characteristics, for outputting a color image corresponding to the halftone-processed image data provided from the halftone processing means. Each of the unit threshold matrixes consists of a plurality of sub-matrixes for not less than one dither threshold plane having at least two color components, an array of relatively low thresholds in the predetermined threshold range corresponding to a tone range of the input image data is equal among the plurality of sub-matrixes, and relatively medium to high thresholds in the predetermined threshold range are arranged aperiodically to extend over series of sub-matrixes.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.